Shutter Island's ending, like the ending of Inception, is effective because it resolves the main plot but does not provide complete closure. It hinges on the question Andrew asked at the end of the movie: if it is worse "to live as a monster, or to die as a good man." Shutter Island is essentially about Andrew falling into a delusion that he is a better man who didn't kill his wife and that his time on earth has meaning because he's looking for a way to avenge her rather than live with the guilt of having killed her.